Research demonstrates that recent stressful life events have a negative impact on mental health. However, the correlation between life event scores and mental health measures are only low to moderate, suggesting substantial individual differences in vulnerability to stressful events. Thus these relatively low correlations may be partially due to the moderating effects of other individual variables. The proposed research tests a congnitive model designed to explain such individual differences in vulnerability. The basic prediction of this model is that greater self-complexity moderates the impact of recent stressful events. In this model, the term self-complexity refers to the number and distinctiveness of the aspects or features that a person uses to cognitively organize knowledge about the self. From a mental health standpoint, the main question of interest is whether self-complexity acts as a protective buffer against the negative mental health consequences of stressful life events. Two preliminary studies supported the prediction that it does. Three studies are proposed. Experiment 1 refines and validates two measures of self-complexity. Experiment 2 is a laboratory study that tests the basic prediction by observing whether self-complexity acts to moderate or buffer reactions to an experimentally manipulated success or failure experience. Experiment 3 is a longitudinal field study that tests the basic prediction by observing whether self-complexity moderates the relationship between mental health and naturally occurring life events. Thus self-complexity should act to moderate or buffer the negative effects of recent stressful events on affect, self-esteem, psychological symptoms, and depression. This longitudinal itme series design provides the opportunity to draw stronger causal inferences regarding the hypothesized moderating effects of self-complexity. The long-term aims of this research are first, to provide insight at a theoretical level into the relationship between cognitive structure and mental health problems; and second, to develop a diagnostic tool for identifying individuals who are highly vulnerable to stressful events or prone to depression.